That's actually exactly what it does. There's a pool of text that has words with 3-4, another with 4-5, then 5-6 then 6-7, then 7-8, then lastly 8-9. The word difficulty increases by the distance to the lane you're trying to type too, and the difficulty also increases with decreasing lives. I've explained more in depth about the letters difficulty in other comments if you want to know more. Thank you for your kind words!